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Description
Seismic reflection data collected south of Santa Rosa and San Miguel Islands provide the basis for a detailed interpretation of the geologic history of the area. High-resolution and intermediate-penetration seismic reflection profiles, limited shallow dart core samples, and adjacent island geology were evaluated. Utilizing these data, an attempt was made to determine structure, geometry, and paleogeographic trends of offshore pre-Pliocene rock units. Five distinct structural features occur within the study area: (1) an E-W-trending acoustic basement high south of San Miguel Island, (2) an anticlinorium southeast of Santa Rosa Island with an axial trend changing from N-S to NW-SE as the anticlinorium merges with the Channel Island structural salient, (3) two grabens south and southwest of Santa Rosa Island with their axes oriented NW-SE, (4) an E-W-trending fault trace which is the probable westward offshore extension of the Santa Rosa Island fault, and (5) a well-developed, discontinuous, en echelon fold pattern trending NW-SE. Four seismically defined units and two high amplitude reflectors have also been recognized. Seismic units are defined on the basis of distinctive internal reflector geometry, and are tied to core control for age date determination. Two paleogeographic trends are suggested by this study: (1) island paleocurrent analysis suggests that one source for Oligocene through Early Miocene (Sespe-Vaqueros) rocks on Santa Rosa Island may be an Eocene-Cretaceous structural high south of Santa Rosa Island. The E-W-trending acoustic basement structure south of San Miguel Island may be a remnant of this southern source area for Sespe-Vaqueros deposition, and (2) seismic stratigraphic columns derived from seismic unit 3 (Middle-Late Miocene age) suggest an offshore extension of channelized midfan, outer fan, and basin submarine fan facies of the Blanca and Monterey Formations.
